The present invention relates in general to simultaneous measurement of liquid temperature and liquid level in boilers for hot water and steam heating.
Immersion thermowells are commonly used to provide a ready means to place temperature sensors in thermally conductive relationship with the liquid in boilers and other liquid vessels. Immersion thermowells are typically supplied with most residential heating boilers and are designed to mount temperature regulating and indication controls to the boilers, as well as offering temperature sensing orifices in the boiler. It is well know by those skilled in the art that immersion thermowells provide a simple means to place sensors in thermal relationship with the fluid in the boiler but also allows easy removal for service without the need to drain the boiler which can be a time consuming and cumbersome task.
Liquid level sensors are not commonly provided by boiler manufacturers due to the significant added cost but are becoming more popular as various US states legislate their use. Specifically, liquid level sensors are used in conjunction with low water cut off limit controls to prevent the boiler from firing when the water level falls below a safe level.
Boiler manufacturers normally do not provide level sensing devices since some states do not require their use. Therefore the installation of low water cut-off controls is typically left to the installer where they are required or desired. The installer may be required to bore holes in the wall of the boiler to provide access to the water, or more commonly will complete the labour-intensive task of routing external piping in the heating system specifically to allow a low water cut-off control to be installed. Homeowners in states not legislating their use are more inclined not to have one installed due to the high cost of installation, despite the obvious added safety that is offered.
In the North America boiler temperature limit controls are subject to the same UL and CSA safety standard as boiler level controls. Since they are subject to the same standard and essentially provide the same function to enable or disable the burner it would be efficient to offer a single control to boiler manufacturers that incorporated both functions in one device. Since virtually all boiler temperature controls mount to immersion thermowells that are designed to only allow thermal conduction of water temperature but preventing the sensors from coming in direct contact with the water it is not practical to provide level sensing means through the immersion thermowells currently available.
The Arekapudi U.S. Pat. No. 5,178,009 discloses an integral temperature and liquid level sensor and control that effectively provide such a solution, but Arekapudi teaches a combination sensor that is permanently fixed to the vessel wall.
John, in U.S. Pat. No. 5,111,691 also provides a single sensor control apparatus that combines liquid level and temperature measurement but also requires the sensor to be mounted directly in the sidewall of the boiler in direct contact with the fluid.
Perry in US patent application publication # 2007/0253463 discloses a means to detect fluid conductivity but requires a plurality of sensors spaced some distance apart.
In U.S. Pat. No. 4,859,076 Twerdochlib discloses a split thermowell for water and steam detection that uses dual temperature sensors and a heating element to detect the presence of water in vessel.